Insights into mechanistic interpretation of crystalline-state reddish phosphorescence of non-planar π-conjugated organoboron compounds

Metal-free room-temperature phosphorescent (RTP) materials are attracting attention in such applications as organic light-emitting diodes and bioimaging. However, the chemical structures of RTP materials reported thus far are mostly predominantly based on π-conjugated systems incorporating heavy atoms such as bromine atoms or carbonyl groups, resulting in limited structural diversity. On the other hand, triarylboranes are known for their strong Lewis acidity and deep LUMO energy levels, but few studies have reported on their RTP properties. In this study, we discovered that compounds based on a tetracyclic structure containing boron, referred to as benzo[d]dithieno[b,f]borepins, exhibit strong solid-state reddish phosphorescence even in air. Quantum chemical calculations, including those for model compounds, revealed that the loss of planarity of the tetracyclic structure increases spin–orbit coupling matrix elements, thereby accelerating the intersystem crossing process. Moreover, single-crystal X-ray structural analysis and natural energy decomposition analysis suggested that the borepin compounds without bromine or oxygen atoms, unlike typical RTP materials, exhibit red-shifted phosphorescence in the crystalline state owing to structural relaxation in the T1 state. Additionally, the borepin compounds showed potential application as bioimaging dyes.


Materials
All reactions were carried out under dry argon.For the reaction solvents, diethyl ether and toluene were purchased from Kanto Chemical Co., Ltd., distilled from calcium hydride, and stored over activated molecular sieves under argon until use.All other chemicals were purchased from FUJIFILM Wako Pure Chemical Industries, Ltd. and TCI Co., Ltd.Starting materials 6, [S1] 1-H, [S2] and 1-F [S2] were prepared according to the literature procedure.Colon26 cells were kindly supplied by Prof. Nagasaki (Osaka Metropolitan University).
The cells were maintained in Dulbecco's Modified Eagle Medium (Thermo Fischer Scientifc, Massachusetts, USA) containing 10% fatal bovine serum (Thermo Fischer Scientific) and antibiotics (Thermo Fischer Scientific).

Analytical Methods
NMR spectra were recorded on Varian System 500 and 400MR spectrometers.Abbreviations Th, Bz, Mes, Tipp, and F Mes used for the following NMR assignments stand for fused thiophene ring, fused benzene ring, mesity group, tripyl group, and 2,4,6-tris(trifluoromethylphenyl) group, respectively.High-resolution mass spectra were obtained by the direct infusion method on a Thermo Fisher Scientific LTQ Orbitrap XL spectrometer at N-BARD, Hiroshima University.UV-vis absorption spectra were measured with a Shimadzu UV-3600 Plus spectrometer.Photoluminescence (PL) spectra, phosphorescence lifetimes, and absolute PL quantum yields were measured with a HORIBA FluoroMax-4 spectrophotometer with an integrating sphere.Fluorescence lifetimes were on a HORIBA DeltaFlex modular fluorescence lifetime system, using a Nano LED pulsed diode excitation source.Variable temperature emission spectra and lifetimes were recorded on a HORIBA Fluorolog-3 spectrofluorometer and corrected for the response of the detector system.

DFT calculations
The S0 geometries were obtained by DFT calculations using a Gaussian 16 program at the B3LYP/6-31G(d) level of theory.The S1 and T1 geometries were obtained by TD-DFT calculations at the same level.All the optimized geometries were verified by vibrational frequency analysis at the same level of theory and found as true minima, as negative vibrational frequencies were not present in all cases.All the TD-DFT calculations were performed for the 10 lowest singlet and triplet states on the S0 and T1 geometries.Spin-orbit coupling matrix elements (SOCMEs) were calculated from TD-DFT results at the B3LYP/6-31G(d) level for the ten lowest singlet and triplet transitions by using the Orca 5.0.3 package software.NEDA calculations were performed on Gaussian 16 with NBO version 7.0.Due to limitations on the number of shells in NBO calculations, it was impossible to perform NEDA calculations for all molecules in the cluster models shown in Figures S15-S17.Therefore, NEDA calculations were conducted for models divided into groups of 2 to 3 molecules, consisting of the central molecule and its surrounding molecules.Finally, the sum of the calculated energies was obtained from the results of each calculation.Dividing the models may not be appropriate when there are strong intermolecular interactions between the divided molecules.However, in the cases studied in this work, since there are no strong intermolecular interactions between the molecules, the division did not significantly impact the computational results.In fact, even when the model was divided with different combinations of molecules, the computational results did not change significantly.

XRD measurements
Single crystal X-ray diffraction data was collected at 100 K on a Bruker AXS SMART APEX II ULTRA diffractometer or on a Rigaku XtaLAB Synergy R, DW diffractometer at Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, using MoKα radiation monochromated with a multilayered confocal mirror.The structure was solved by Intrinsic Phasing on the SHELXT-2014/5 program or by the CrysAlis Pro and expanded using Fourier techniques.Non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were included but not refined (SHELXL-2017/1).All other calculations were performed using the APEXII crystallographic software package of Bruker AXS or using the OLEX2 program.Graphical crystal structures were generated using Mercury 4.3.1 (Cambridge Crystallographic Data Centre).

Preparation of water-dispersible H-Si-Mes and pyrene
The crystal of H-Si-Mes or pyrene were co-incubated with P123 in MilliQ (H-Si-Mes or pyrene 0.5 mg; P123, 4.5 mg) and the resulting solution was sonicated (60 W, 5 min) with probe type sonicator on ice.After removal of precipitation by centrifugation (4 o C, 10000 g, 15 min), the dispersibility was confirmed by measuring UV-Vis absorption spectrum.We further addressed size of the complex of H-Si-Mes with P123 by dynamic light scattering (DLS) measurement (Zeta Sizer Nano, Malvern, UK).In addition, their electrochemically negative character was also revealed by ζ-potential measurement.Morphological observation was carried out by transmission electron microscope (TEM, JEM-1400, JEOL Ltd.Co., Japan).The samples were casted on the grid and stained with 4 % phosphotungstic acid.The stained samples were observed by TEM (acceleration voltage, 100 keV).

Bioimaging
Murine colon carcinoma cells were exposed to the complex of H-Si-Mes with P123 for 24 h.After washing with PBS, the medium were exchanged with fresh medium.The samples were observed by confocal laser scanning microscopy (LSM700, Carl Zeiss, Germany).

H-H-Tipp
Table S1 Optical data of X-H-Ar in the solid state.

Figure S1
Figures and Tables

Figure S2 Figure S3
Figure S2Fluorescence spectra of H-Si-Mes before and after argon bubbling in THF at room temperature.

Figure S4 Figure S5
Figure S4 Photographs of recrystallized crystals of benzo[d]dithieno[b,f]borepins taken under room light or

Figure S6 Figure S7
Figure S6 Phosphorescence decays of benzo[d]dithieno[b,f]borepins in the solid state at room temperature.

Figure S8
Figure S8Photographs of H-Si-Mes_C R taken under UV (365 nm) in air and vacuum.

Figure S9
Figure S9PL spectra of crystals of X-H-Ar in air at room temperature.

Figure S10
Figure S10 Optimized structures of benzo[d]dithieno[b,f]borepins with dihedral angles of C=C-C=C in S0 ,

Figure S14
Figure S14 Optimized structures of model compounds in S0 geometry at the B3LYP/6-31G(d) level of theory.

Figure S16
Figure S16Relative phosphorescence spectra of H-Si-Mes and 1-H in 2-MeTHF at 77 K.The intensity is

Figure S20 Figure S22 Figure S23
Figure S20 ORTEP drawings and packing diagrams of F-H-F Mes.

Figure S24
Figure S24 Absorption spectra (A) and absorbance changes at 343 nm (B) of H-Si-Mes complexed with P123

FigureFigure S30 H
Figure S25 (A) Representative morphology of H-Si-Mes with P123 observed by transmission electron

Figure S31 C
Figure S31 C NMR spectrum of H-H-Tipp in CDCl3 at room temperature (126 MHz).

Figure S32 B
Figure S32 B NMR spectrum of H-H-Tipp in CDCl3 at room temperature (160 MHz).

Figure S34 C
Figure S34 C NMR spectrum of F-H-Tipp in CDCl3 at room temperature (126 MHz).

Figure S35 B
Figure S35 B NMR spectrum of F-H-Tipp in CDCl3 at room temperature (160 MHz).

Figure S37 C
Figure S37 C NMR spectrum of H-H-F Mes in CDCl3 at room temperature (126 MHz).

Figure S38 B
Figure S38 B NMR spectrum of H-H-F Mes in CDCl3 at room temperature (160 MHz).

Figure S40 C
Figure S40 C NMR spectrum of F-H-F Mes in CDCl3 at room temperature (126 MHz).

Figure S41 B
Figure S41 B NMR spectrum of F-H-F Mes in CDCl3 at room temperature (160 MHz).

Table S5 TD
-DFT results of model compounds at the S0 geometries at the B3LYP/6-31G(d) level.

Table S8
Emission lifetime of H-Si-Mes_C R at various wavelengths under 100 K.
Figure S18 ORTEP drawings and packing diagrams of H-Si-Mes.

Table S9
Solution properties of complex of H-Si-Mes with P123.